Device for increasing contrasts at tone value leaps and contours in printing forms



Nov. 29, 1960 H. TAUDT 2,962,548

DEVICE FOR INCREASING CONTRASTS AT TONE VALUE LEAFS AND CONTOURS INPRINTING FORMS Filed Aug. 12, 1958 Unite DEVICE FOR INCREASING CONTRAS'IS AT TONE VALUE LEAPS AND CONTOURS IN PRINTING FORMS Heinz Taudt,Kiel, Germany, assignor to Dr.-Ing. Rudolf Hell Kommanditgesellschaft,Kiel-Dietrichsdorf, Germany, a German company This invention isconcerned with a method of and apparatus for artificially increasingcontrasts at tone value leaps and contours in printing forms produced bymeans of electronically controlled machines employing scanning ofsurrounding fields. The term tone value leaps is intended to mean suddenor abrupt changes in the tone value of areas or regions of picturecopies to be reproduced.

In reproduction methods for relief printing of pictures, there occurs alessening or diminution of contrasts and sharpness with respect toindividual tone value ranges and also at the borders of abrupt tonevalue transitions and contours, such lessening being mainly caused bythe screening which is indispensable for the printing. This lessening ordiminution of contrast and sharpness which becomes manifest in that thecourse of the transition from black to white or white to black extendsin the print at the border of tone value leaps and contours moregradually than in the original, so that a narrow gray zone appears inplace of a sharp border line or contour line, respectively, and further,in that black areas contain small white dots while white areas containsmall black dots.

It often becomes desirable to eifect changes in the reproduction, forexample, in the case of picture copies which are difficult to reproducedue to lacking contrasts or sharp contours, requiring falsification ofthe print as compared with the original copy to provide the desiredimprovement.

It is known in connection with modern electronic machines for producingprinting forms or plates, in which the depth of penetration of anengraving tool into the material of a printing form blank is controlledby the brightness of photoelectrically scanned picture points of thecopy to be reproduced, to impart to the gradation, that is, to thecorrelation between the brightness and the blackening of the print andthe brightness and blackening of the copy, within certain limits adesired course, making it possible to effect an increased or diminishedresolution of the black, gray or white tone value ranges and even toeffect a tone value reversal without preparatory alteration (retouching)of the copy or subsequent alteration (partial corrective etching) of theprinting form. This gradation alteration, however, effects all pictureportions wherever identical blackening is encountered, Without givingpreference to sudden tone value leaps and contours. It is moreover knownto make these gradation alterations locally variable and, by means ofscanning surrounding areas, dependent upon the character of the localtone value, so that different picture portions with the same blackshading are reproduced with different gradation.

The television technique employs for the improvement of the horizontalresolution of the television picture the method of differentiatingdistortion correction. For the compensation of the flattening of picturesignals at a sharp tone value change, there is thereby achieved a atnt r2,962,548 Patented Nov. 29, 1960 ice steepening of the transition in thesignal, by subtracting from the picture signal the second differentialquotient thereof. The eflective operating range of this correction maybe determined by the choice of the amplitude of the second derivation ofthe picture signal.

In accordance with the invention, an artificial contrast increase attone value leaps or contours is obtained by reproducing in the immediateneighborhood on both sides of the tone value leap and the contour,respectively, the darker picture portion darker and the brighter pictureportion brighter than at a greater distance from the corresponding leapor contour, respectively. This local tone value control produces anarrow demarcation line or seam (gloriole) on both sides of the tonevalue leap and the contour, respectively, which emphasizes the suddenchange. The human eye tolerates such falsifications and recognizes themas increasing contrasts provided the emphasis is held within certainlimits, that is, provided it is not overdone.

The invention is realized by deriving a difference signal from thepicture signal and the surrounding area signal and additively ormultiplicatively influencing the picture signal by the differencesignal.

The various objects and features of the invention will be brought out inthe course of the description which will be rendered below withreference to the accompanying drawing, in which Fig. 1 shows thescanning light dot or spot and the surrounding field upon passing asudden tone value change or leap;

Fig. 2v illustrates the course of the picture and surrounding fieldsignal voltage on both sides of the tone value leap;

Fig. 3 indicates the course of the difference signal voltage;

Fig. 4 shows the course of the picture signal affected by the differencesignal; and

5 is a circuit for carrying out the desired correctron.

In Fig. 1, numeral 1 indicates the scanning light dot or spot which maybe in the form of a small circle, square or rectangle, with a diameteror length of the sides, respectively, on the order of the spacingbetween two neighboring screen lines. This small light dot or spotserves in known manner to scan the picture copy to be reproduced pointby point in successive lines and to ascertain the brightness of theindividual picture points photoelectrically. The fluctuating photocurrents released in the photocell incident to illumination thereof bythe reflecting scanning light, control in known manner the depth ofpenetration of the engraving tool into a printing form blank,corresponding to the fluctuating brightness of the scanned picturepoints of the copy to be reproduced. Numeral 2 indicates the circularauxiliary scanning light area for scanning the immediate neighborhood ofthe picture point, such area being conveniently referred to assurrounding field. The auxiliary scanning light area is larger than thatcovered by the scanning light spot 1 and surrounds such spotconcentrically or eccentrically. The scanning of the surrounding fieldof the picture point does not determine details of the copy to bereproduced, but the average or mean (integral) brightness closelyadjacent to the picture point being scanned. Numeral 3 indicates theborder or demarcation line between a darker picture portion 4 and abrighter picture area 5, it being assumed for the sake of simplicitythat the tone values of the corresponding portions remain constant inthe close vicinity of the border line 3. The border line 3 usually willnot be rectilinear but may be assumed to be rectilinear for a small partof the length which lies within the order of magnitude of the diameterof the surrounding field. It shall further be assumed that the scanninglight spot 1 and the auxiliary scanning light area 2 move during thescanning operation perpendicular to the border or demarcation line 3 inthe direction of the arrow from the left to the right across the line 3.The scanning direction will generally not extend transverse to thecontour. If it is oblique to the contour, the conditions will not changemuch; the tone value leap will then merely be passed over somewhatslower. If the scanning is effected for a portion parallel to or alongthe contour, the latter will be gradually passed over steplike due tothe operation of the transverse advance of the scanning systemperpendicular to the scanning direction after the scanning of each line.The following observations are independent of the angle obtained betweenscanning direction and contour.

Fig. 2 shows in graphic representation the course of brightnessresulting from the point or spot and from the surrounding field scanningor the photoelectric signal voltage U proportional thereto, in the closevicinity on both sides of the tone value leap, dependent upon the motionS of the scanning system. Curve 6 represents the course of the picturesignal voltage produced by the scanning of the tone value leap by meansof the scanning light spot 1. The voltage curve, in accordance with theassumed constant brightness, extends at the left of the changehorizontally and leaps at 3 to a higher constant value corresponding tothe assumed higher constant brightness at the right of the leap. Thesteepness of the voltage leap depends upon the sharpness of thetransition of the two tone value regions at 3. Curve 7 represents thecourse of the surrounding field voltage which is produced by thescanning of the tone value leap by means of the auxiliary scanning lightarea 2 (surrounding field). This voltage curve which corresponds to themeans or average (integral) brightness in the close vicinity of thescanned picture point, extends initially likewise horizontally,thereupon rising in S-shape due to the circular configuration of thesurrounding field, the rise beginning prior to and spaced from the leap3 and ceasing spaced from the leap in back thereof, the spacingcorresponding to the diameter of the surrounding field. At the change orleap point 3, the surrounding field signal voltage is equal to the meanvalue lying between the two constant voltages. It is with respect to thepicture signal voltage higher directly ahead of the voltage leap andlower directly in back thereof.

In Fig. 3, the curve 8 rep-resents the course of the diiference signalvoltage which is obtained when the surrounding field signal voltage issubtracted from the picture signal voltage. The difference signalvoltage which is initially zero, becomes negative at the point at whichthe surrounding field signal voltage begins to increase, assumes itsminimum at the leap point, passes through zero at the center point ofthe leap, reaches maximum at the end of the leap, and drops to zero atthe point at which the surrounding field signal voltage ceases toincrease.

The curve 9 shown in Fig. 4 represents the course of the picture signalvoltage (curve 6 in Fig. 2) upon being affected additively ormultiplicatively by the difference signal voltage according to curve 8,Fig. 3, the additive or multiplicative modification being effected bysuperimposition or by using the difference signal as a regulationvoltage for amplifying the picture signal. The curve 9 is at the startof the leap at a minimum which lies under the lower constant picturesignal value at the left of the leap and is at the end of the leap at amaximum lying above the higher constant picture signal value at theright of the leap.

The consequence is that the engraving tool cuts into the material of theprinting form blank at the beginning of the leap to a lesser depth thanshortly before while cutting thereinto at the end of the leap somewhatdeeper than shortly thereafter. This means that in the printed pictureproduced by means of the completed printing form, the black at the leftof the one value leap will be 4 somewhat blacker than the correspondingblack in th original and that the white at the right of the end of thetone value leap will be somewhat whiter (insofar as this is possible)than the corresponding white on the original which is being reproduced.It is, however, the purpose of the correction to bring out or emphasizeexactly the small (flat or dull) tone value leaps since the large tonevalue leaps which are rich in contrast are naturally easily recognizableand clearly visible. The resuit is that there will appear at each sideof the tone value leap a narrowly extending respectively somewhatblacker or somewhat whiter demarcation line or seam (gloriole) whichincreases artificially the originally small contrast between the twomutually bordering tone value ranges.

Fig. 5 shows in the form of a block diagram a circuit arrangement forcarrying out the correction.

Referring now to Fig. 5, numerals 1 and 2 respectively indicate thescanning light point or spot and the auxiliary or surrounding scanningfield. The dash lines indicate in schematic manner the connections ofthe two optical scanning systems with the photocells 10 and 11 whichrespectively ascertain the brightness of the scanning point 1 and thatof the surrounding field 2. Details of apparatus for carrying out thecorresponding scanning operations are omitted to avoid unnecessarilyencumbering disclosure; suitable devices being described, for example,in German Patents Nos. 930,491 and 949,443. The picture signal and thesurrounding field signal may be produced either spatially by means oftwo different photocells or according to time by interlacing the twosignals, or electrically by the use of difierent carrier frequencies. Inthe latter two cases, only one photocell Wlll be needed. Accordingly,the two signals will either be Initially spatially separated or theyWlll be separated according to time by periodically controlled switchesor, lastly, electrically by means of filters.

It is customary to chop the scanning light for thepicture point and forthe surrounding field periodically either by the use of rotatingperforated disks, to produce one carrier frequency or two differentcarrier frequencies, or else to modulate a carrier frequency or twodifferent carrier frequencies with the direct current signals producedby the scanning operation in the two photocells 10 and 11, becausealternating voltages are more easily controlled for amplificationpurposes. The photocells 10 and 11 are accordingly connectedrespectively with alternating current preamplifiers 12 and 13, thusproducing an amplifier channel for the picture signal and for thesurrounding field signal, respectively. These channels will beconveniently referred to respectively as picture channel and assurrounding field channel.

The picture channel comprises the preamplifier 12, a regulationamplifier 24, an amplitude filter 25, and further, not illustratedswitching means, for example, a gradation regulator, a superimposingstage for supplying the screen or raster frequency, a rectifier,calibration means for the adjustment of the black level, the white leveland the screen amplitude and, finally, an amplifier for controlling thedrive of the electromagnetic engraving system. These further switchingand control devices are not absolutely required for the understanding ofthe operation of the circuit which takes place in the initial stage.

The surrounding field channel comprises the amplifier 13 and, if needed,switching means 15 and 17. There is also provided a branch channelcomprising, if needed, switching means 14 and 16 and, finally, theditference channel comprising subtraction switching means 18, regulationamplifiers 19 and 2.0, amplitude filter 21, potentiometer 22, and switch23. The regulation amplifier 19 is controlled from the picture channeland the regulation amplifier 20 is controlled from the surrounding fieldchannel.

The outputs of the amplifiers 12 and 13 are connected by way of theswitching means 14, 15, 16, 17, which will be presently explained, withthe'two inputs of the subtraction switching device 18 in which thesurrounding field signal voltage is subtracted from the picture signalvoltage. The subtraction may be carried out employing the alternatingvoltages of the signals. This presupposes, in order to avoid errors,that the two alternating voltages are completely alike in phase, thatis, that they have the identical frequency, that they originate in thesame carrier frequency generator, and that there are, up to thesubtraction, no switching means that would displace the phases thereofin different manner. Since this can hardly be accomplished withoutauxiliary expenditure, it will be advisable to rectify the twoalternating signal voltages prior to subtraction, in the rectifiers 16and 17, which is also advantageous since the two signal voltages arealso to be employed as regulation voltages. The subtraction may beelfected in simple manner, for example, by connecting the two signalvoltages in opposition. It may be necessary in some circumstances toinsert distortion members 14 and 15 for the picture signal and for thesurrounding field signal, respectively, ahead of the subtraction stage18, such distortion members causing a tone value change (gradationvariation) which may be different for each member.

The difference signal is amplified in the regulation amplifier 19, whichis regulated by the picture signal, and thereupon again in theregulation amplifier 20, which is regulated by the surrounding fieldsignal. The amplitude filter 21, following the regulation amplifiers l9and 20, limits the regulated difference signal in either directionthereof.

The amplified and limited difference signal i conducted to theregulation input of the regulation amplifier 24, disposed in the picturechannel, by way of the potentiometer 22 which may be used for adjustingthe regulated and amplitude-limited difference signal vo tage, and byway of the disconnect switch 23 serving for disconnecting the differencechannel if desired. The am lifier 24 is connected with the am litudefilter 25 serving for limiting the regulated and corrected picturesignal in either direction.

The following observations will aid in understanding the operation ofthe system.

In the bright picture regions, the demarcation line or seam may bewhiter than the brightest white of the picture, whereby the picturepoints would be undercut in the seam. This may be disturbing in the caseof subsequent uniform auxiliary etching that may be required in somesituations in connection with metallic printing forms. Similarly, in thedarkest picture regions, the seam might be blacker than the blackestshade of the picture to be reproduced. The consequence of this lattercondition would be that the engraving tool would not at all affect theprinting form b-ank. the black seam accordingly remaining unscreened andwithout small white dots. This is generally not disturbing but might beundesired in some circumstances. To remedy this condition, there isprovided the am litude filter 25 which suppresses picture voltagescorresponding to tone values which are whiter than normal white andblacker than normal black.

The greater the tone value leap, the stronger will be the differencesignal. However, the intention is to bring out the small tone valueleaps since the great leaps are naturally well visible. Assumingproportional action of the difference signal in the regulation amplifier24, the demarcation lines or seams would become over-emphasized in thecase of great tone value leaps. The amplitude filter 21 in thedifference channel serves for limiting the difference signals.

It might be desirable in some cases to effect corrections only in onedirection or stronger in one direction than in the other, or to affectthe direction and degree of non-symmetry depending upon the brightnessof the picture point or of the surrounding field or both, for example,to effect correction in the black region only in the direction ofbrighter tone values (only the black seam) but no correction in thewhite content in the direction of darker tone values (no white seam) orvice versa (only white seam; no black seam) and at the same timeeffecting correction in the intermediate tones in both directions fromthe darker to the brighter as well as from the brighter to the darkertone values (black seam and also white seam). For these purposes thereare provided in the difference channel the regulation amplifiers 1a and20 which are respectively affected by the picture signal and by thesurrounding field signal.

The following applications are possible:

For example, if the shading and light parts lack contrast in the picturecopy to be reproduced, the gradation for increasing the lacking contrastmay be set by adjusting the tone value stage in the picture channel (inback of the limiter 25) so as to effect stronger resolution of shadesand lights which, however, causes diminished resolution of theintermediate tones. The loss of intermediate tones which thus occurs iscompensated at small tone value leaps by artificial contract increase inthe seams by increased effectiveness of the dlfierence signals withinthe region of the intermediate tones. The difference signal is therebyalways less amplified with respect to shades and lights. An improvedshade and light reproduction is in this manner produced by setting ofthe gradation and improved intermediate tone reproduction is effectedfrom the diiferencechannel. The appearance of tone values which arerespectively blacker than the normal black and whiter than the normalwhite on the picture copy, and exaggerated corrections due to great tonevalue leaps are prevented by the action of the limiters 21 and 25.

In case the picture copy to be reproduced lacks contrast in the lightcontents, proper setting of the gradation in the picture channel willresult in increasing the resolution of the light content while at thesame time reducing the shades and the intermediate tones. The loss onshades and intermediate tones will again be compensated at the tonevalue leaps in the corresponding tone value regions, by increasedefiectiveness of the difference signals producing artificial contrastincrease at the seams. The dilference signals are thereby graduallyalways less amplified in the direction of the light portions. This willresult in improved reproduction of the light parts due to the setting ofthe gradation and improved shade and intermediate tone reproduction bycontrol from the difference channel.

If the shade portions are too dull and fiat in the picture copy to bereproduced, the resolution of the shades may be increased and that ofthe light parts and intermediate tones may at the same time bediminished by suitable setting of the gradation in the picture channel.The loss in light portions and intermediate tones is again compensatedat the tone value leaps in the corresponding tone value regions, due toincreased effectiveness of the difference signal, causing artificialcontrast increase at the seams. The difference signal is thereby alwaysgradually less amplified in the direction of the shade parts. Thisresults in improved shade reproduction under control of the gradationand improved light and intermediate tone reproduction by control fromthe difference channel.

Finally, if the intermediate tones in the picture copy to be reproducedare too soft, the resolution of such tones may be increased withsimultaneous decrease of the resolution of the shades and lights, byproper setting of the gradation in the picture channel. The loss inshades and lights will again be compensated at the tone value leaps inthe corresponding tone value ranges due to increased etfectiveness ofthe difference signals, by artificial con trast increase in the seams.The difference signal is thereby always gradually less ampified fromboth directions to the intermediate tones, resulting in improvedintermediate tone reproduction and improved light and shade reproductionby the action of the difference channel.

The action of the surrounding field channel upon the picture channel maybe set to a desired value once for all, or may be calibratable,connectible and disconnectible, or regulatable continuously or instages. These possibilities are indicated in Fig. by the potentiometer22 and the switch 23 which are disposed between the limiter 21 and theinput of the regulation amplifier 24.

In the production of color extract forms for threeor four-colorprinting, the correction procedure described herein, in order to avoidundue complications will be limited to the blue and to the blackextraction, respectively, since these extractions determine thecontours.

In some instances, for example, in the case of picture copies withcoarse paper structure or photographs with coarse granulation,over-emphasis of such structure, in the reproduction, may be avoided bythe use of a scanning light spot with a diameter or length of the sidethereof greater than normal, for example, two to three times the linespacing. In spite of the consequent primary loss of sharpness, therewill result an advantage and improvement in the reproduction due to theemphasis of the tone value leaps.

In case of producing, in an electronically controlled machine, printingforms with variable reproduction scale, that is, forms which areenlarged or reduced as compared with the copy to be reproduced, thereare provided shutters or diaphragms disposed in the path of the rays forthe picture spot scanning and for the surrounding field scanning,respectively, such diaphragms serving the purpose of adjusting the sizeof the picture point or spot and of the surrounding field, which are tobe scanned, in accordance with the variable spacing of the scanninglines.

The light surface covered by the surrounding field may be very large ascompared with the scanning light spot for the picture point, forexample, two to three times the size corresponding to the adjacentsharpness range of the light spot; it may also be sharply limited or mayhave a blurred or indistinct margin; and its light intensity may beconstant or may diminish toward the border thereof. All thesepossibilities are provided for so as to adapt the surrounding fieldscanning to the diverse correction problems and requirements to be metin practice.

Changes and modifications may be made within the scope and spirit of theappended claims in which is defined what is believed to be new anddesired to have protected by Letters Patent.

I claim:

1. In an electronically controlled machine for producing artificiallyincreased contrast at tone value leaps and contours in printing formshaving engraved thereon the contents of copies to be reproduced whereinthe engraving is effected responsive to photoelectric scanning of thecopy to be reproduced, the combination of means for line by linephotoelectric scanning of the copy to be reproduced by a light spot,means for simultaneously photoelectrically scanning a field surroundingsaid light spot, means for deriving a difference signal from the signalsresulting respectively from the light spot scanning and from thesurrounding field scanning, and means utilizing said difference signalfor additively or multiplicatively affecting the signals resulting fromthe light spot scanning to provide in the immediate vicinity on bothsides of a tone value leap and of a contour dark copy portions darkerand bright copy portions brighter than they appear on the copy to bereproduced.

2. A device according to claim 1, comprising means operatively relatedto said light spot scanning means for distorting the signals resultingfrom the light spot scanning prior to deriving said difference signals.

3. A device according to claim 1, comprising means operatively relatedto said light spot scanning means for distorting the signals resultingfrom said surrounding field scanning prior to deriving said differencesignals.

4. A device according to claim 1, comprising means operatively relatedto both of said scanning means for additively or multiplicativelyaffecting the difference signals by the signals resulting from saidlight spot scanning and by the signals resulting from the surroundingfield scanning.

5. A device according to claim 1, comprising means operatively relatedto said combining means for limiting the amplitude of the differencesignals.

6. A device according to claim 1, comprising means for continuouslyregulating the effect of the difference signals upon the signalsresulting from the light spot scanning.

7. In a system for producing, with the aid of an electronicallycontrolled machine, printing forms having en graved thereon the contentsof copies to be reproduced, wherein the engraving is effected responsiveto line-byline photoelectrical scanning of a copy to be reproduced by alight spot to ascertain point for point the relative tone values of saidcopy, a device for artificially increasing contrasts at tone value leapsand contours appearing in the copy to be reproduced, said device beingoperative to reproduce respectively in the immediate vicin'ty on bothsides of a tone value leap and of a contour dark copy portions darkerand bright copy portions brighter than they appear on the copy to bereproduced, said device comprising means for simultaneouslyphotoelectrically scanning a field surrounding said light spot toascertain the mean tone value of such surrounding field, means forming apicture channel including a preamplifier and a regulation amplifier andan amplitude filter, means for'conducting to the input of said picturechannel the signals resulting from said light spot scanning whichsignals are to be corrected, a picture branch channel extending fromsaid picture channel in back of said preamplifier and containing signaldistortion means and a rectifier, a surrounding field channel forreceiving the signals resulting from the scanning of said surroundingfield and comprising an amplifier and signal distortion means and arectifier, and a difference channel comprising in serial relationship asubtraction device, a first and a second regulation amplifier, anamplitude filter, a potentiometer, and a switch, said subtraction devicehaving two inputs for respectively receiving signals from said picturebranch channel and from said surrounding field channel for subtractingthe latter signals from the former signals, means for connecting saidfirst regulation amplifier with said picture branch channel and saidsecond regulation amplifier with said surrounding field channel, andmeans for connecting said switch with the regulation amplifier disposedin said picture channel.

References Cited in the file of this patent UNITED STATES PATENTS2,777,058 Boyajean Jan. 8, 1957 2,880,270 Hell Mar. 31, 1959 2,892,887Hell June 30, 1959

